array_expert_insert_reduce.js

const elements = [1, 2, 4, 5];
const indexToInsert = 2; // we will append the element at index 2
const elementToInsert = 3; // we will insert the number "3"

const nextElements = elements.reduce((previousValue, currentValue, index) => {
  previousValue.push(currentValue); // 😱
  if (index === indexToInsert) {
    previousValue.push(elementToInsert); // 😱😱
  }
  return previousValue;
}, []);